The present invention relates to an image sensor for use in hard-copy image reading devices such as facsimiles, scanners and so on and relates to a method of manufacturing the same image sensor.
Recently, there has been an increasing demand for further improved and miniaturized one-dimensional image sensors for converting image information into electric signals in conjunction with an increasing demand for image-reading apparatuses such as facsimiles and image scanners. Among the existing one-dimensional image sensors, there are two predominant types, one of which is a contract type optical image sensor using mirror and the other is a contact image sensor using a rod-lens array. A contract type image sensor using a micro-lens array and an optical waveguide array has been last developed.
FIG. 1 is illustrative of a system of a conventional contract type image sensor. Light from an LED array 1 striking a surface of an original 2 is reflected to a mirror 22 wherefrom it is further reflected several times between mirrors 22. After this, the reflected light passes a lens 23 and forms an image on a light-receiving element array 5. Thus, the image information can be obtained.
FIG. 2 is illustrative of a conventional contact type image sensor using a typical rod-lens array 24. Light from an LED array 1 striking a surface of an original 2 is reflected and fall into a rod-lens array 24 through which it passes and forms an image on a light-receiving element array 5. This device can be miniaturized owing to its simple construction.
FIG. 3 is illustrative of a contract type image sensor using a conventional micro-lens array 3 and optical waveguide array 25. Light from an LED array striking a surface of an original 2 is reflected to a micro-lens array 3 including the specified number of equally spaced micro-lenses corresponding to respective pixels of a specified image format. The converged light passes the optical waveguide array 25 and forms an image on a light-receiving sensor array 5. The device can be thus miniaturized. Furthermore, it is possible to precisely induce rays of light from the micro-lens array into the optical waveguide array 25 by matching numerical aperture NA of the micro-lens array 3 with that of the optical waveguide array 25. This is disclosed in Japanese Laid-Open Patent Publication No. 9-37038.
The contract type image sensor of FIG. 1, however, necessarily contracts a whole image information to {fraction (1/10)} in size through a lens. Therefore, the sensor must have an elongated conjugate length. For example, a distance from an original document of B4 in format size to a light-receiving element may be about 330 mm. An optical path is usually folded by mirrors 22. However, the conjugate length of the device is above 70 mm. This makes it difficult to further reduce the size of the device.
The contact type image sensor using a rod-lens array 24 as shown in FIG. 2 can have a considerably reduced distance between an original document and a light-receiving element by using a short-focusing lens since whole image information is formed in the same scale. The latest model has the document-to-element distance of about 10 mm. However, the device of this type has a very shallow depth of field of, e.g., 0.5 mm at which its MTF can be maintained at a level higher than 40% for the specification of 200 dots per inch. Application of a sensor having a shallow depth of field in a handy scanner or a flat head scanner may arise a problem that an uneven surface portion of a double spread page of a book or pasted slips cannot be read. To avoid this, a rod-lens having a small numeral aperture (NA) and a deep focal depth must be used. However, the use of such a rod-lens may elongate the conjugate length of the device. For example, a rod-lens array 24 having a depth of field of about 3 mm has a conjugate length of about 50 mm resulting in increasing the size of the device. In addition, the rod-lens array having a deep depth of field is expensive.
The compact type sensor may have a depth of field of 6 mm or more on the same conditions.
As described above, both types of sensors involve such a common problem that miniaturization of the device is always accompanied by shallowing a depth of field of the lens array while the use of the lens array having a deep depth of field necessarily increase the size of the device. In other words, these two factors are tradeoffs to each other.
There has been developed an image sensor (FIG. 3) having a micro-lens array and an optical waveguide array, which has a 20 mm distance between an original document (B4 in size) and a light-receiving element. As disclosed in Japanese Laid-Open Patent Publication No. 9-37038, this image sensor is constructed in such a way that the micro-lens array 3 and the optical waveguide array 25 have the same numeric aperture and the image forming size of the former matches the core size of the latter. This feature can considerably reduce the crosstalk from neighboring elements of micro-lens array 3 and neighboring pixels. However, this arrangement may cause another problem that an image being in out-of-focus or as contacted in size due to a floating area of the original document may increase crosstalk and cause a considerable change in quantity of light due to the out-of-focus position of the image. In addition, this device causes an image information from the original to be contracted through the curved optical waveguide array and, therefore, may be necessarily increased in its general size as the number of image pixels increases. The device is more difficult to be miniaturized than the contact type sensor using a rod-lens array. It is also noted that elongation of the optical waveguide array is accompanied by increasing loss of light therein.
Accordingly, an object of the present invention is to provide an image sensor that includes an array of a specified pixel-corresponding quantity of micro-lenses arranged for forming an image of light reflected from an original document, an array of the specified pixel-corresponding quantity of optical fibers or waveguides arranged for transmitting an image formed by the micro-lens array as respective optical signals of pixels, and an array of light-receiving elements for receiving optical signals from the optical fiber or waveguide array and converting the signals into electric signals, wherein the optical fiber array or optical waveguide array has cores whose end-faces have each a size smaller than a size of an image formed by corresponding micro-lens and are located on the micro-lens-side from the image forming plane when the original document is placed at a given position. The construction of this image sensor is such that an image information of the original is transmitted pixel by pixel through pairs of micro-lens with an optical fiber or waveguide to respective light-receiving elements. This may reduce crosstalk due to floating of the original from the base level and contraction of its image through the micro-lens array if the image is formed in size larger than the core of the optical fiber or waveguide when the original document is placed at a given position. The image formed thus being in out of focus can be read at its center portion by the optic fiber array whose core-end-faces are located on the micro-lens side from the image-forming plane. This means that floating of the original may approach the image-forming plane to the core-end-faces of the optical fiber array, thereby the image becomes improved with a reduced variation of light quantity. As the result of the above-described effects, an image finally obtained through the light-receiving element array can have a deep depth of field.
Another object of the present invention is to provide an image sensor that includes an array of a specified pixel-corresponding quantity of micro-lenses arranged for forming an image of light reflected from an original document, an array of the specified pixel-corresponding quantity of optical fibers arranged for transmitting an image formed by the micro-lens array as respective optical signals and an array of light-receiving elements for receiving the optical signals from the optical fiber array and converting them into electric signals, wherein the optical fiber array has the optical fibers arranged parallel to each other therein. This image sensor attains a very compact and effective construction allowing a light signal from the micro-lens array to be transferred to the light-receiving element array through a shortest path with a minimum loss of light. Furthermore, the sensor is inexpensive since its optical fiber array is suitable for mass production at a low cost by the same method as manufacturing the rod-lens array and is easy to be coupled with the light-receiving element array owing to a wide pitch between the arrays.
Another object of the present invention is to provide an image sensor that uses an optical fiber array formed by arranging optical fibers each composed of a core and a clad in an array, bonding them to a jig with adhesive containing nontransparent material having a refractive index higher than that of the clad and further providing light-shield film covering incident (input-side) end-faces of the optical fibers excluding core end-faces. This image sensor can shut off light falling on the end-faces of the clads, which may become stray light if it was allowed to enter therein. A part of incident light, which has entered each core at a larger numeric aperture NA, may be refracted into the clad and then absorbed by an adhesive layer having a higher refractive index. Furthermore, this array can be formed from conventional optical fibers each composed of a core and a clad.
Another object of the present invention is to provide an image sensor that uses an optical fiber array formed by arranging optical fibers each composed of a core and a clad having a refractive index being, at its inside, lower than that of the core and gradually increasing toward its outside surface and then adhering the optical fibers to a jig with adhesive containing nontransparent material having a refractive index higher than that of that of the outside surface of the clad. This image sensor can shut off light falling on the clad, which may become stray light if it enters therein. Furthermore, a part of incident light, which enters the core at a larger numeric aperture NA and is refracted into clad, can be lead toward the outside of the clad and absorbed by an adhesive layer containing a substance having a higher refractive index. This optical fiber array has no need for provision of further shielding for shutting of f light falling on the clads such as light shielding film.
Another object of the present invention is to provide an image sensor using an optical fiber array in which optical fibers each having a three-layer structure composed of a core, a clad and an absorbing layer, with a refractive index of the clad sandwiched between the core and the absorbing layer is lower than those of the core and the absorbing layer, are arranged in an array and fixed to a jig with adhesive containing nontransparent material. In this sensor, stray light may enter into the clad, hit the absorbing layer and is completely absorbed and attenuated therein since the absorbing layer has a higher refractive index than that of the clad. This optical fiber array dose not require any further protection against stray light such as light shielding film.
Another object of the present invention is to provide an image sensor that uses a jig with optical fibers adhered thereto and having a thermal expansion coefficient substantially equal to that of the light-receiving elements to be coupled therewith. This image sensor can be free from occurrence of misalignment (pitch error) between the optical fiber array and the light-receiving element array owing to the same level of thermal expansion.
Another object of the present invention is to provide a method of manufacturing an image sensor, which comprises steps of: putting a transparent substrate on an end-face of an optical fiber array or an optical waveguide array, applying an ultraviolet-curing resin-coat on the substrate, forming thereon cylindrical islands each having a diameter corresponding to a region of divergent angle of the optical fibers or optical waveguides by ultraviolet radiation from the reverse-side end-face and forming spherical lens surfaces of the respective islands by thermal reflowing and simultaneously adjusting the transparent substrate thickness to a value at which each micro-lens being shaped to the effective region of diverging radiation of each optical fiber or each optical waveguide can have a diameter smaller than that of a pixel. The manufacturing method can easily produce a micro-lens array that has element-to-element pitches corresponding to those of the optical fibers or optical waveguides. This image sensor has no need of optical alignment of micro-lens array with optical fiber or waveguide array.
Another object of the present invention is to provide a method of manufacturing an image sensor, which comprises steps of: putting a transparent substrate on an end-face of an optical fiber array or an optical waveguide array, applying a photosensitive resin layer on the substrate, forming thereon swellings each having a diameter corresponding to a region of divergent angle of the optical fibers or the optical waveguides by ultraviolet radiation from the reverse-side end-face and forming semi-spherical lens surfaces of the respective islands by the effect of allowing therein unreacted monomers. At the same time, the thickness of the transparent substrate is adjusted so that each micro-lens to be shaped to the effective region of diverging light of each optical fiber or each optical waveguide may have a diameter smaller than that of a pixel. The manufacturing method can easily produce a micro-lens array whose element-to-element pitch corresponds to that of the optical fiber array or the optical waveguide array. This method also assures realizing the above-mentioned effect.